1. Field of the Invention
The present invention relates to a low-pass filter and a voltage-current conversion circuit used in the same, and more particularly to a low-pass filter inserted into D/A (Digital to Analog) converted output of an audio apparatus.
2. Description of the Related Art
In recent years, an audio signal has been changed from the analog form to the digital form. In an output circuit, however, the signal is changed from a digital audio signal to an analog audio signal by a D/A converter. A large amount of pulse noise generated during the D/A conversion is included in the analog audio signal, and in order to remove this, a circuit called VLSC (Vector Linear Shaping Circuitry) (registered trademark of Onkyo Corporation) is provided.
A low-pass filter used in the VLSC is disclosed in Japanese Unexamined Patent Publication No. 2003-283299 (Document 1). In this low-pass filter, a voltage-current conversion circuit is used (refer to FIG. 2 of Document 1), wherein four resistors (R31 to R34 in FIG. 2 of Document 1) are connected between input and output thereof. Therefore, an increase in a combined resistance value of these resistors is unavoidable (about 50 kΩ in a circuit for practical use), and thermal noise increases, thereby reducing a signal-to-noise ratio (S/N).